ISLAMABAD (Reuters) - Pakistani Interior Minister Nisar Ali Khan said on Thursday he would quit his post once the Supreme Court has ruled on corruption accusations against Prime Minister Nawaz Sharif, whatever the verdict.

The announcement by one of Sharif's longstanding political allies marks a fresh blow for the premier as he awaits a ruling, expected on Friday, that could disqualify him from office.

Khan said he had clashed with members of Sharif's Pakistan Muslim League-Nawaz (PML-N) over how to tackle the Panama Papers leak, which led to an investigation into how Sharif obtained his wealth.

Sharif this month dismissed as slander a report from the corruption investigation that raised questions about his wealth.

"The day Supreme Court decides - whether it is in favor of Nawaz Sharif or against him - I have decided I will quit my ministry and I will also quit the National Assembly," Khan said.

Khan said his heart was no longer in politics and he wanted to end his 33-year association with PML-N.

"I'm done with politics," Khan told a news conference.

Khan's importance to PML-N over the years has been enhanced by his close ties with Pakistan's powerful military. Khan's elder brother once served as chief of the army general staff and his brother-in-law was also a powerful general.

Analysts say Khan has often acted as a bridge between the military and Sharif, who has had tumultuous relations with the army during his three stints in power.

SUPREME COURT VERDICT

Khan praised Sharif's decency during his news conference, but he lashed out against sycophants close to the premier and spoke of a conspiracy to exclude him from meetings.

"I had the courage to tell the truth in front of Nawaz Sharif no matter how bitter that truth was," Nisar said.

Political analyst Rasul Baksh Rais said the Supreme Court's decision would make or break the PML-N's fortunes in elections due next year, but added that Khan's departure would have little impact.

"If the ruling goes against Mr. Nawaz Sharif, then the Muslim League-N is not going to maintain its coherence, its public appeal. It will lose and the winner will be ... (opposition leader) Imran Khan" said Rais, who teaches politics at the Lahore University of Management Sciences.

Any ruling short of disqualification would mean "politics as usual", he added.